In such systems, the tank venting system has a tank venting valve in a regeneration line which connects a storage container collecting fuel vapors from a fuel tank with an intake pipe of an internal combustion engine, a stop valve for sealing the tank venting system such that it is air-tight relative to an atmosphere prevailing outside the motor vehicle, and a bistable pressure switch, the switched state of which indicates whether a predefined switching pressure is exceeded or not reached in the tank venting system.
In the usual methods used to verify the tightness of a tank venting system while a motor vehicle is in motion, as described in DE-19713085-A1 for example, the tank venting valve is opened so that the negative pressure in the intake pipe can disperse in the tank venting system. Once the tank venting valve has been subsequently closed, the pressure should remain approximately at the level of the negative pressure reached. Both the extent of the negative pressure reached as well as the period until this negative pressure is reached and the behavior of the pressure after the tank venting valve is closed enable conclusions to be drawn regarding a possible leak in the tank venting system. A pressure sensor is required in order to be able to continually observe and monitor the pressure curve.
By way of contrast, various methods for verifying a possible leak in a tank venting system which can be performed when the engine is switched off are known from DE 102 45 158 A1. In one of these methods, the drop in the engine coolant temperature after the warm engine is switched off is observed. If there is a sufficiently small temperature, it is assumed that the fuel tank has also cooled and that the pressure prevailing in the tank venting system has also dropped. At the same time, a vacuum switch is monitored which is triggered at a certain negative pressure. If the vacuum switch has not been triggered even though the temperature has already dropped sufficiently, it is suggested that there is a leak in the tank venting system.
The advantage in this method lies in that a pressure sensor is no longer required. The considerably more reasonably priced vacuum switch or pressure switch reduces the cost of the method. However, the comparatively long waiting time of up to several hours until the temperature has dropped sufficiently to be able to come to a reliable conclusion about the existence of a leak is disadvantageous.